Automobile exhaust gases are conventionally purified with a catalyst supported on a ceramic body able to withstand high temperatures. The preferred catalyst support structure is a honeycomb configuration which includes a multiplicity of unobstructed parallel channels sized to permit gas flow and bounded by thin ceramic walls. The channels can have any configuration and dimensions provided gases can freely pass through them without being plugged by entrained particulate material. Examples of such preferred structures include the thin-walled ceramic honeycomb structures described in U.S. Pat. Nos. 3,790,654 to Bagley and 3,112,184 to Hollenbach.
Generally similar ceramic structures are used as diesel engine particulate filters. In such applications, ceramic honeycomb filters are fitted to diesel engine exhaust systems for removal of particulates from the high temperature diesel engine exhaust gases. Examples of diesel engine particulate filters are disclosed in U.S. Pat. Nos. 4,329,162 to Pitcher, Jr. and 4,415,344 to Frost et al. Again, the ceramic materials utilized in such applications must have a high thermal shock resistance and a low coefficient of thermal expansion.
Cordierite (2MgO.2Al.sub.2 O.sub.2.5SiO.sub.2) is known to display a very low thermal expansion over a wide range of temperatures. In substantial amounts, cordierite gives a ceramic body excellent thermal shock resistance when subjected to rapid and severe changes in temperatures. This property has caused cordierite to find widespread use as a catalyst support for automotive catalytic converters and as diesel engine particulate filters.
Typically, when used as a catalytic converter substrate, the cordierite body is treated with a high surface area alumina washcoat in the form of a slurry of suspended alumina. The alumina washcoat serves as a support for the later-applied catalyst material. The washcoat is applied, for example, by submersing the cordierite substrate in or passing the substrate through a stream of the alumina slurry. The amount of alumina coating deposited on the surface of the substrate is directly proportional to the substrate's ability to absorb water. The amount of alumina deposited, in turn, has a direct effect on the amount of catalyst that will eventually be deposited on the substrate. Catalyst materials coated on converter substrates are typically very expensive. Automobile manufacturers often require varying levels of water absorption depending on the particular catalytic converter and the level of performance required. This enables the automobile manufacturer to achieve a desired catalytic efficiency while minimizing waste from coating excess, expensive catalyst material on the substrate.
Cellular ceramic bodies absorb water due to the negative pressure (capillarity) developed in the body's pore structure. In a typical ceramic body used as a catalytic substrate, pores will vary widely in size and are interconnected with direct access to the surface of the body. Increasing or decreasing the total porosity will, in turn, increase or decrease water absorption, respectively.
Relatively minor changes in the raw materials (e.g., particle size or morphology of the precursor material) used in a cordierite body are known to influence the pore structure of the resultant body. Such changes in the precursor materials commonly occur from batch to batch. Over time, these minor changes can lead to a drift in the water absorption capability of the body. In the past, the drift has been corrected by adding an entirely new mineral compound to the batch composition. This compound is added as required to drive the water absorption back within acceptable limits.
Typically, compounds such as silica have been used for this purpose. However, silica has many disadvantages. It is very difficult to distribute silica uniformly in the batch due to the small amounts conventionally used. In addition, use of silica requires modifications in the amounts of other ingredients to maintain the proper analytical batch composition effective to produce a cordierite phase upon firing. Therefore, there is a need for procedures to adjust water absorption of cordierite bodies without the disadvantages discussed above.